Devices utilizing nonlinear optical effects have a number of applications such as up and down frequency converters, parametric oscillators and amplifiers. These devices are based on the nonlinear optical properties of certain nonlinear optical (NLO) materials. Three commonly used NLO materials are LiNbO.sub.3, LiTaO.sub.3 and KTP. In a frequency converter such as a frequency doubler, a bulk crystal of NLO material is cut and polished to allow input and output beams to propagate at a specific angle with respect to the crystal lattice. Frequency converted (e.g., doubled) light is created from the input beam. The relatively low conversion efficiencies of these devices, however, have limited their usefulness. Conversion efficiency is, among other factors, greatly affected by the coefficient of nonlinearity of the material used. The coefficient of nonlinearity is also very important for the other applications of NLO materials mentioned above.
It has been found that poling the crystal structure of such NLO materials can greatly enhance the desired nonlinear optical (NLO) effects. Poling consists of forming patterned regions within the NLO material that have predetermined molecular polarizations. This phenomenon has stimulated great interest in the development of patterned poled NLO material technologies. Periodically poled LiNbO.sub.3, for example, exhibits a factor of 20 conversion efficiency improvement over bulk LiNbO.sub.3 when used as a frequency converter. The other NLO material applications mentioned above are similarly affected. Patterned poled LiNbO.sub.3 is discussed here because it has recently been developed to a level where it is useful for practical devices.
The problem is that patterned poled NLO materials are only available in very limited thickness, at least for the foreseeable future. Poled LiNbO.sub.3, for example, is currently limited in thickness to 1 mm. This is a result of the best process known for making patterned poled LiNbO.sub.3 ; electric field poling. Reference can be made to Myers et al., Journal of the Optical Society of America, 12(11), p.2102-2116 concerning electric field poling techniques. This process starts with a thin plate of single crystal LiNbO.sub.3, perhaps 0.5 mm.times.10 mm.times.10 mm in size, and for this discussion, oriented horizontally. The optic axis is oriented perpendicular to the plane of the plate (vertical), an orientation commonly referred to as "Z-cut" in the art. An electrode pattern is lithographically fabricated on the top surface of the plate. For a periodically poled pattern as may be used in a frequency doubling device, the spatial period of the poling (and therefore the electrodes) may be from 2-30 microns. The electrodes are connected to a high voltage power source, which supplies an electric field across the plate in selected regions. The polarity of the applied electric field is selected to oppose the ferroelectric polarization of the material. When voltage is applied such that the electric field inside the plate exceeds the coercive field of the material, the domain polarity of the material between the electrodes will flip 180 degrees to reorient with the applied field. The accurately patterned metal traces provide accurate control over which regions will become domain inverted. The critical coercive field for domain inversion for LiNbO.sub.3 is quite high at 21 KV/mm. This field is, unfortunately, close to the dielectric strength. When attempts are made to pole thicker plates of LiNbO.sub.3, slight irregularities in the plate thickness or defects in the crystal cause dielectric breakdown, and destroy the sample. Even in cases where dielectric breakdown does not occur, the domain pattern fidelity in thick plates degrades because of the high aspect ratio that must be maintained through a thick crystal Similar effects limit the thickness of other patterned poled NLO materials. The electric field poling technique does have the great advantage of allowing precise patterns to be poled over a large surface area.
Bulk periodically poled LiNbO.sub.3 with large apertures has been fabricated directly by modulating the crystal growth process, but it suffers from long range uniformity problems that significantly degrade device performance. Another disadvantage of this method is that it can only make one-dimensional patterns. This precludes the construction of improved and novel devices which can be constructed with higher dimensional domain inversion patterns. A further disadvantage of this technique is that the conditions required for domain modulation are detrimental to high quality crystal growth and compositional homogeneity.
Thicker plates are of interest because the greatest nonlinear effect is achieved when the interacting light beams propagate perpendicular to the optic axis, i.e., in the plane of the plate. Thus, the useful aperture of poled NLO devices is severely limited by the thickness of the poled plates. The small aperture also limits the optical power handling ability of a poled device because NLO material has a damage threshold. For LiNbO.sub.3, for example, the damage threshold is 3J/cm.sup.2 for a 10 ns, 1064 nm optical pulse. Any increase in optical intensity (power/area) above this threshold will damage the NLO device. Thus, beams cannot simply be focused down to compensate for the small aperture of patterned poled NLO devices; damage will result. Since a process for producing accurate poling in thick bulk NLO materials does not currently exist, a scheme is needed for increasing the useful aperture of patterned poled NLO devices.
A number of methods exist for increasing the aperture of poled NLO material. The following discussion focuses on periodically poled plates because they are commonly used in laboratory NLO applications and are commercially available.
Using an elliptical beam shape to enter the edge of a poled plate increases the beam footprint which allows more power at a given intensity. A practical limit for the asymmetry of the major and minor axes of the elliptical beam is possibly 5:1 due to the difficulty of elliptical focusing. This increase is useful, but still will not meet power handling requirements in many applications. The elliptical beam profile may require elaborate beam shaping optics, though in some cases, the elliptical shape may actually match that of the laser pump source as in the case of a diode laser, for example.
FIG. 1 illustrates a possible arrangement called a face pumped geometry. This geometry provides the desired NLO effects but at significantly reduced efficiency compared to end pumping as in FIG. 3. When the plate is face pumped, the grating vector is not parallel with the beam, which results in decreased NLO effects. Also, the interaction length is now limited by the thin dimension of the plate. This further reduces the NLO effect. The smaller the angle between the grating vector and the beam, the less efficiency degradation. However, since NLO materials typically have high refractive indices, a larger external angle of incidence is required to achieve a given internal angle. The large angle of incidence results in high surface reflection losses.
Another prior art solution is shown in FIG. 2. In this drawing, the plate is in the plane of the paper. This arrangement consists of wedged ends on the periodically poled plate, formed for example by cutting the ends at an angle or bonding prisms to the end faces (as shown), to expand the beam input side and recompress the beam on the output side. This method has the advantage of allowing the beam to travel parallel to the grating vector of the plate. A problem with this solution is that it only expands the incoming beam in one dimension, limiting the increase in aperture size. Further, the wedged surface increases the difficulty of aligning beams through the thin plate.
Another disadvantage of current methods of fabricating domain-patterned materials is that there is no capability for three-dimensional patterning. The techniques of growing bulk domain-patterned materials by modulating growth parameters allows the ability to modulate only in one dimension, i.e. along the growth axis. The techniques of electric field poling, which is the current favored approach to fabricating domain patterned materials, allows two-dimensional patterning because the electrode structure is formed by a lithographic mask design. Patterning in the third dimension is not possible because the domain pattern is constrained to follow the ferroelectric axis of the material, so the pattern defined by the surface electrode is reproduced through the depth of the material. The potential benefits of three-dimensional domain patterning can be inferred from the two-dimensional domain-patterned devices that have been successfully demonstrated in the current technical literature. The extension to three dimensions has not been demonstrated because of the unavailability of a device with three-dimensional patterned structures.
Laser rods with large apertures have been created by bonding together a number of smaller laser rods. These devices utilize lasing medium materials and not NLO materials. Plates of dissimilar NLO materials have been bonded to produce waveguide structures and quasi-phasematched structures. These devices do not provide for increased power handling capability and preclude the use of electric field patterning.
Therefore there is a need for novel structures providing increased aperture size, increased and three-dimensional patterning capability. Future applications of poled plates depend upon the development of such structures.